This invention relates to a combined cycle power plant comprising a combination of a gas turbine plant and a steam turbine plant.
A combined cycle power plant is a power generation system comprising a combination of a gas turbine plant and a steam turbine plant. It performs the required work shared between a gas turbine for a high temperature range of thermal energy and a steam turbine for its low temperature range. Because of this configuration, this system effectively recovers and utilizes thermal energy. This power generation system has attracted marked attention in recent years.
On the combined cycle power plant, research and development have been performed with one of the crucial considerations for efficiency improvement being how far the high temperature range can be raised.
To form a high temperature range, a cooling system has to be installed from the aspect of heat resistance of a turbine structure. Air has been used as a coolant for the cooling system.
As long as air is used as a coolant, some disadvantages are encountered even if a high temperature range could be achieved. That is, a power loss is caused for pressurizing air, required for cooling, to a necessary pressure by means of an air compressor component of the system. In addition, the air that was used for cooling of the parts is finally introduced into a turbine channel through which a hot gas passes. As a result, the average gas temperature is lowered to decrease the energy of the gas. In view of these disadvantages, a further improvement in thermal efficiency cannot be expected in such a configuration.
In an attempt to solve these problems and achieve a further efficiency improvement, a system has appeared which adopts steam as a substitute for air as a coolant for the gas turbine. For instance, Japanese Unexamined Patent Publication No. 4210/95 has proposed such a system.
FIG. 3 shows the essential part of the system disclosed in Japanese Unexamined Patent Publication No. 4210/95, which has the following constitution:
A combined cycle power plant comprises a gas turbine plant 5 mainly composed of a gas turbine 1, an air compressor 2 and a combustor 3; an exhaust heat recovery boiler 10 using an exhaust gas from the gas turbine plant 5 as a heating source and mainly composed of a high pressure steam generator area 11, an intermediate pressure steam generator area 12, a low pressure steam generator area 13, and a reheater 14; and a steam turbine plant 20 supplied with steam from the exhaust heat recovery boiler 10 and mainly composed of a high pressure turbine 21, an intermediate pressure turbine 22, and a low pressure turbine 23.
A cooling system incorporated in this combined cycle power plant is a steam cooling system. According to this system, bleed steam extracted halfway in the stages of the high pressure turbine 21 is used as cooling steam. This steam is guided through a steam supply path 24 to a steam cooling system component 30 provided in a high temperature cooled area of the gas turbine 1 to cool this high temperature cooled area.
The cooling steam that cools the high temperature cooled area of the gas turbine 1 in this manner is itself heated by this cooling. That is, the steam receives thermal energy from the high temperature cooled area of the gas turbine 1. Then, the heated steam is further heated during passage through the reheater 14, and fed to the intermediate pressure turbine 22 of the steam turbine plant 20, where its accumulated thermal energy is recovered.
The numeral 25 denotes a condenser. Condensate obtained there is sent by a pressure pump 26 to the exhaust heat recovery boiler 10 via a feed water pipe 27. Part of the condensate is branched, and mixed into the aforementioned bled gas, which is cooling steam, from the high pressure turbine through a water spray pipe 29 by a feed water pump 28 to bring the cooling gas to a low temperature.
The numeral 6 denotes a generator, which is driven by the aforementioned gas turbine plant 5 and steam turbine plant 20.
In the conventional power plant constituted as described above, bleed steam extracted halfway in the stages of the high pressure turbine 21 is used as cooling steam which is guided to the steam cooling system component 30 provided in the high temperature cooled area of the gas turbine 1 to cool this high temperature cooled area. The amount of this bleed steam is not necessarily sufficient for use as cooling steam.
Unless the pressure loss of cooling steam in the high temperature cooled area, including the steam cooling system component 30, is made sufficiently small, therefore, a cooling system cannot be realized.
If the flow rate of the cooling steam is insufficient, moreover, the temperature of the steam in the high temperature cooled area becomes high. Thus, a spray or the like is needed to agree this high temperature with the temperature of a downstream part, for example, the inlet temperature of the intermediate pressure turbine. This leads to a decrease in the efficiency of the plant.
The path of the cooling steam constitutes an in-line channel. Should a leak of the cooling steam occur on the upstream side, therefore, the blade on the downstream side may fail to be supplied with cooling steam, and may be damaged.